There's no need to cut into the panel. You can just make an adapter to go from an 8" down to a 6.5". The easiest way to upgrade those is to just get the Kicker/Shelby door subs. Then if you're going with an external amp, personally, I'd love to get the Image Dynamics 5x7's that fit right in the stock location but sound awesome. I'm not sure if I'm getting an external amp, though. I'm not quite sure if I'm ready to tear the interior out of a brand new car.
I agree with you, don't cut into a new car. I installed Orion Cobalt 5x7 in the doors and rear deck. Just installed an MB Quart Onyx 4 channel amp. I have a PAC signal convertor, one with four sensitivy controls. Know I need to send time to adjust the sound levels. At least I have the wiring done.